Rotary and linear actuators exist in various forms to provide rotary or linear positioning of valves, dampers, linkages, etc. in industrial applications. Most of these actuators position the load under closed or open looped control. Some applications require the movement of the load to a "home position" when the process is shut down at the end of the day, shift, or production run of a product. Electronic or other means of cycling the actuator to the "home position" before shut down is relatively easily accomplished. However, if the electric power source serving the actuator and other devices is interrupted, damage to the process can quickly occur if the valve damper or linkage is not shut down.
Moving the load to the "home position" in a controlled and safe manner without the presence of electrical energy is difficult to accomplish by virtue of the few devices available. The means of storing energy must pick up or be transferred to the load in a smooth manner to have the minimal detrimental impact to the process and the equipment involved, including the mechanics of the actuator. After picking up and beginning movement, the actuator must accelerate the load to a reasonable velocity and have adequate energy to get the load to the "home position." After the load is positioned at the "home position," the actuator must have a means of safely dissipating or storing the excess energy. This condition is complicated by the fact that the load is an unknown. It could be at the upper limit of the actuator's capacity or it could be zero in the case of the actuator being cycled to a fully extended or retracted position inadvertently without a load and then having the line power interrupted. Thus, the actuator should be able to effectively handle a full rated load to a zero load condition. Also, line power is often lost for only a few electrical cycles. If the actuator needs to be reset or run through a "cocking" sequence (which preloads the mechanism) following a loss of line power, the process being controlled can easily be upset even though the loss of line power was only of a few milliseconds duration.